The present invention relates to a ferroelectric liquid crystal material. More particularly, the present invention is concerned with a ferroelectric liquid crystal composition having a quick response despite its small spontaneous polarization value which comprises a non-chiral smectic liquid crystal compound and an optically active compound, and an optical switching element comprising the same.
2. Description of the Prior Art
Liquid crystal compounds are widely used as a material for display devices. Most of these display devices are used in a TN-type display system, and liquid crystal materials belonging to a nematic phase are used for this purpose.
Since the TN type display system is a non-emissive display type, it has advantages such as freedom from occurrence of eyestrain and very low power consumption, but on the other hand it has drawbacks such as slow response and difficulty in viewing of display depending upon viewing angle. In recent years, this type of system has been converted in such a manner that features in the form of a flat display could be utilized. In particular, quick response and wide viewing angle have been demanded.
In order to meet the above demand, an attempt has been made to improve the liquid crystal material. However, the TN display system is considerably inferior to other emissive types of display (e.g., electroluminescent display, plasma display, etc.) in response time and width of viewing angle.
Therefore, development of a novel liquid display system as an alternative to the TN type display system is indispensable to attain a combination of utilization of the features of a liquid crystal display device, such as non-emissive display type and low power consumption, with attainment of response characteristics comparable to those of the emissive display. As one of such attempts, N. A. Clark and S. T. Lagerwall have proposed a display system wherein an optical switching phenomenon of a ferroelectric liquid crystal is utilized [see Appl. Phys. Lett., Vol. 36, p. 899 (1980)].
The existence of ferroelectric liquid crystals has been first published in 1975 by R. B. Mayer [see J. Phys., Vol. 36, p. 69 (1975), and these ferroelectric liquid crystals belong respectively to a chiral smectic C phase, a chiral smectic I phase, a chiral smectic F phase, a chiral smectic G phase, a chiral smectic H phase, a chiral smectic J phase, and a chiral smectic K phase (hereinafter simply referred to as "S.sub.C * phase", "S.sub.I * phase", "S.sub.F * phase", "S.sub.G * phase", "S.sub.H * phase", "S.sub.J * phase" and "S.sub.K * phase", respectively) from the viewpoint of the structure of the liquid crystal.
When the optical switching phenomenon of the ferroelectric liquid crystal is applied as a display device, it exhibits two features superior to those of the TN-type display system. The first feature is its very quick response, and the response speed is one thousand to ten thousands higher than that of the TN-type display system. The second feature is the memory effect, and this facilitates multiplexing drive through cooperation with the above-described quick response.
Particular attention is now focused on the Sc* phase among the chiral smectic phases.
When the ferroelectric liquid crystal is applied to a display device, it should meet the following requirements.
(1) It should exhibit an Sc* phase over a wide temperature range including room temperature (at least 0.degree. to 50.degree. C.).
(2) A response time of 100 .mu.sec or less is required for a display device of 640.times.400 lines or more.
The response time (.tau.) to the electric field of the ferroelectric liquid crystal is expressed by the following equation: ##EQU1## wherein .eta. is the viscosity, Ps is the spontaneous polarization and E is the electric field strength. For this reason, in order to realize a quick response, it is necessary to have a large spontaneous polarization value.
However, in recent years, occurrence of abnormal phenomena accompanying the large spontaneous polarization value has been reported (see, for example, Akio Yoshida et al., The 13rd Symposium on Liquid Crystals, 142-143 (1987); J. Dijon et al., SID 88 DIGEST, 246-249 (1988); or H. R. Dubal et al, Jpn. J. Appl. Phys., 27 (1988) L2241-L2244). According to these reports, when a commercially available polyimide film is used as an insulating layer, the spontaneous polarization value should be 30 nC/cm.sup.2 or less, preferably 25 nC/cm.sup.2 or less.
(3) According to N. A. Clark, in order to attain the memory effect, a helix should be unwound by making the value of a cell gap (d) smaller than the pitch value (P) of the helix (see Appl. Phys. Lett., 36, 899 (1980). For this reason, it is necessary to lengthen the pitch of the helix of the ferroelectric liquid crystal for the purpose of enabling the use of a cell having a large thickness cell gap which can be easily prepared.
(4) The oriented state of a ferroelectric liquid crystal varies depending upon the phase series of the liquid crystal. At the present time, it is considered that a liquid crystal having a smectic A phase and a cholesteric phase (hereinafter simply referred to as "S.sub.A phase" and "N* phase", respectively) on the high temperature side of an Sc* phase exhibits the best oriented state through aligning techniques (surface treatment processes) currently used in the TN liquid crystal material. In other words, it is desired that the ferroelectric liquid crystal have the following phase series: isotropic liquid (hereinafter simply referred to "Iso").fwdarw.N*.fwdarw.S.sub.A .fwdarw.S.sub.C * (see, for example, Japanese Patent Application Laid-Open No. 250086/1986).
Further, it is considered that among the liquid crystal materials having the above-described phase series, those having a larger pitch value in the N* phase exhibit a better oriented state (see, for example, Japanese Patent Application Laid-Open No. 255323/1986).
Besides the above-described requirements, there are various requirements such as tilt angle (.theta.) of the liquid crystal molecule.
Even when only the temperature range is taken up, only a few ferroelectric liquid materials can be put to practical use. Therefore, at the present time, there are few ferroelectric liquid materials which can meet all the above-described requirements and can be put to practical use.
For example, Japanese Patent Application Laid-Open No. 291679/1986 and a pamphlet of PCT International Publication No. WO86/06401 each disclose a ferroelectric liquid crystal comprising a mixture of a non-chiral phenyl pyrimidine compound having a smectic C phase (hereinafter simply referred to as "S.sub.C phase") with an optically active compound and describe that this ferroelectric liquid material exhibits an S.sub.C * phase over a wide temperature range including room temperature and has the phase series Iso.fwdarw.N*.fwdarw.S.sub.A .fwdarw.S.sub.C * and has a spontaneous polarization value of 30 nC/cm.sup.2 or less. Although the ferroelectric liquid crystal composition described in the above-described publications satisfies the above-described requirements in respect of the temperature range of the S.sub.C * phase, phase series and spontaneous polarization value, it cannot be put to practical use because the response time is 300 to 500 .mu.sec (see, for example, ferroelectric liquid crystal compositions described in Examples 1 and 2 of Japanese Patent Application Laid-Open No. 29167/1986 or ferroelectric liquid crystal compositions described in Examples 45 and 46 of the pamphlet of PCT International Publication No. WO86/06401).
Japanese Patent Application Laid-Open No. 541/1988 discloses a ferroelectric liquid crystal composition comprising a mixture of a non-chiral phenyl pyrimidine compound with an optically active compound represented by the general formula (III), (IV) or (V) of the present invention and describes that this ferroelectric liquid crystal composition exhibits an S.sub.C * phase over a wide temperature range including room temperature and has a response speed as high as 100 .mu.sec or less. Although the ferroelectric liquid crystal composition disclosed in the above-described patent application satisfies the above-described requirements with regard to the temperature range of the S.sub.C * phase and response time, it cannot be put to practical use due to the difficulty in attaining the excellent orientation attributable to the absence an N* phase (see, for example, Examples 1, 3 and 5), and occurrence of an abnormal phenomenon when a switching operation is conducted attributable to a spontaneous polarization value of 30 nC/cm.sup.2 or more (see, for example, Example 2).
As is apparent from the foregoing description, at the present time, there are few ferroelectric liquid crystal materials which can satisfy all the above-described requirements and can be immediately applied to a display device.